1) Field of the Invention
The present invention relates to a method and apparatus for detecting a deterioration of a catalyst converter disposed within an exhaust gas passage of an internal combustion engine having two air-fuel ratio sensors upstream and downstream of the catalyst converter.
2) Description of the Related Art
Generally, in a feedback control of the air-fuel ratio in a single air-fuel ratio sensor (O.sub.2 sensor) system, a base fuel amount TAUP is calculated in accordance with the detected intake air amount and detected engine speed, and the base fuel amount TAUP is corrected by an air-fuel ratio correction coefficient FAF which is calculated in accordance with the output signal of an air-fuel ratio sensor (for example, an O.sub.2 sensor) for detecting the concentration of a specific component such as the oxygen component in the exhaust gas. Thus, an actual fuel amount is controlled in accordance with the corrected fuel amount. The above-mentioned process is repeated so that the air-fuel ratio of the engine is brought close to a stoichiometric air-fuel ratio. According to this feedback control, the center of the controlled air-fuel ratio can be within a very small range of air-fuel ratio around the stoichiometric ratio required for three-way reducing an oxidizing catalysts (catalyst converter) which can remove three pollutants CO, HC, and NOx simultaneously from the exhaust gas.
In the above-mentioned O.sub.2 sensor system where the O.sub.2 sensor is disposed at a location near the concentration of an exhaust manifold, i.e., upstream of the catalyst converter, the accuracy of the controlled air-fuel ratio is affected by individual differences in the characteristics of the parts of the engine, such as the O.sub.2 sensor, the fuel injection valves, the exhaust gas recirculation (EGR) valve, the valve lifters, individual changes due to the aging of these parts, environmental changes, and the like. That is, the characteristics of the O.sub.2 sensor fluctuate, or if the uniformity of the exhaust gas fluctuates, the accuracy of the air-fuel ratio correction amount FAF is also fluctuated, thereby causing fluctuations in the controlled air-fuel ratio.
To compensate for the fluctuation of the controlled air-fuel ratio, double O.sub.2 sensor systems have been suggested (see: U.S. Pat. Nos. 3,939,654, 4,027,477, 4,130,095, 4,235,304). In a double O.sub.2 sensor system, another O.sub.2 sensor is provided downstream of the catalyst converter, and thus an air-fuel ratio control operation is carried out by the downstream-side O.sub.2 sensor in addition to an air-fuel ratio control operation carried out by the upstream-side O.sub.2 sensor. In the double O.sub.2 sensor system, although the output characteristic V.sub.2 of the downstream-side O.sub.2 sensor shown in FIG. 1B has a lower response speed when compared with the output characteristic V.sub.1 of the upstream-side O.sub.2 sensor shown in FIG. 1A, the downstream-side O.sub.2 sensor has an advantage in that the output fluctuation characteristics are small when compared with those of the upstream-side O.sub.2 sensor, for the following reasons:
(1) On the downstream side of the catalyst converter, the temperature of the exhaust gas is low, so that the downstream-side O.sub.2 sensor is not affected by a high temperature exhaust gas.
(2) On the downstream side of the catalyst converter, although various kinds of pollutants are trapped in the catalyst converter, these pollutants have little affect on the downstream-side O.sub.2 sensor.
(3) On the downstream side of the catalyst converter, the exhaust gas is mixed so that the concentration of oxygen in the exhaust gas is approximately in an equilibrium state.
Therefore, according to the double O.sub.2 sensor system, the fluctuation of the output of the upstream-side O.sub.2 sensor is compensated by a feedback control using the output of the downstream-side O.sub.2 sensor.
In the above-mentioned double O.sub.2 sensor system, however, when the catalyst converter is deteriorated, the downstream-side air-fuel ratio sensor may be affected by unburned gas such as HC, CO, and H.sub.2, thereby also deteriorating the output characteristic V.sub.2 thereof as shown in FIG. 1C. In this case, the controlled air-fuel ratio is fluctuated by a feedback control by the downstream-side air-fuel ratio sensor, thus also deteriorating the fuel consumption, the driveability, and the conditions of the exhaust emission characteristics for the HC, CO, and NOx components thereof.
Accordingly, a technique has been proposed of observing the deterioration of the catalyst converter when the amplitude of the output signal from the downstream-side O.sub.2 sensor is larger than a predetermined value, the period of the output signal from the downstream-side O.sub.2 sensor is smaller than a predetermined value, or a ratio of the period of the output of the upstream-side O.sub.2 sensor to the period of the output of the downstream-side O.sub.2 sensor is larger than a predetermined value.
In this technique, however, the catalyst converter can be judged as deteriorated even when the output characteristics of the upstream-side O.sub.2 sensor are deteriorated, as shown in FIGS. 2A to 2C indicating the output of the upstream-side O.sub.2 sensor, air-fuel ratio correction amount, and the output of the downstream-side O.sub.2 sensor, respectively, when the upstream-side O.sub.2 sensor is in a normal state, FIGS. 3A to 3C indicating the output of the upstream-side O.sub.2 sensor, air-fuel ratio correction amount, and the output of the downstream-side O.sub.2 sensor respectively when the upstream-side O.sub.2 sensor is in an abnormal state, and FIG. 4 indicating the O.sub.2 storage effect of the catalyst converter. In this condition, an amplitude of the air-fuel ratio correction amount FAF becomes larger as shown in FIG. 3B and the air-fuel ratio A/F fluctuates beyond the controllable window W.sub.1 of the air-fuel ratio as shown in FIG. 4, whereby non-purificated exhaust gas is exhausted even though the catalyst converter is in normal state. In this way, the amplitude of the output of the downstream-side O.sub.2 sensor becomes large and the period thereof becomes small, similar to the condition when the catalyst converter is deteriorated, so that the catalyst converter is erroneously judged to be deteriorated.